What kind of Relay Reacts to Ohms?

Question

I need to connect a relay to my old truck to turn on the "soon to add" electric fans.

The ECM has an engine coolant temperature sensor that uses the output resistance to determine coolant temperature.

From the service manual, I have this data:

If you can not see the pic above:

__Condition________________|_Engine coolant temperature decided
Just as ignition switch is | 
turned ON or Start_________|_20°C (68°F)_______________________
More than 6 minutes after  | 
ignition ON or Start_______|_80°C (176°F)______________________
                           | 20 - 80°C (68 - 176°F)
Except as shown above______| (Depends on the time)_____________

So, is there a way to get some sort of Relay that would turn a switch ON when Resistance translates to a certain value (like 185°F or so)?

EDIT More Pics!

Answer 1

If you don't need a lot of precision in your circuit, you can build a simple voltage divider as follows

^{simulate this circuit – Schematic created using CircuitLab}

If you calculate the values in such as way that the voltage at output reaches the relay threshhold voltage, then when R1 get low enough to signal high temperature, the relay driven off the output will switch on. I think most mechanical relays are not very precise with respect to on-volate, so I would consider a solid-state relay for this.

As an example, lets say you want your fan to turn on above 50 degrees (1k sense resistence), and your relay operates at 5V. Given that for a voltage divider \$V_{out}\ = \dfrac{R_{2}}{R_{1}+R_{2}}\$, we just plug in all the values to get the unknown:

5V = \$\dfrac{1K}{1K + R_2}*12V\$

\$R_2\$ = 1.4K, so you use a 1.5k Ohm resistor. The current losses in this voltage divider are only 4.8mA, so it isn't like you are throwing away a lot of power as heat. On the other hand, your triggering resistance is 1K, so at 12V you can only supply 12mA of current to the relay. This will be insufficient for most mechanical relays, but should be OK for solid state relays (SSRs).

Answer 2

In general, environmental sensors are just that. They consist of some material that reacts to the particular parameter being sensed, e.g. temperature, pressure, humidity, etc. In most cases, the sensor material reacts to change an electrical quantity. In the case of a thermistor, it is resistance versus temperature. For a thermocouple, it is voltage versus temperature. For a pressure sensor, it is often resistance (usually in a bridge configuration) versus pressure. In all of these cases, additional circuitry is used to process the material change to convert it into something more useful for either displaying the measured quantity and/or using it to perform a function such as switching a relay or turning on an indicator light, etc. This additional circuitry is needed because most sensors do not have enough energy output to directly power a display or a relay. Also, the output of most sensors is not linear and/or have an offset so mathematical manipulations are needed to convert it into useful information such as temperature in degrees or pressure in psi,etc. These days, the output of most sensors eventually inputs to a processor which decides whether the indicated value requires doing something like driving a relay to turn on fan and/or setting an audible or visible alarm.